Various materials may be used to absorb or dissipate energy from an impact, vibration or load which would otherwise be transmitted to an underlying structure or body. Such materials are used in a wide variety of applications where such absorption or dissipation is desirable, for example, in sporting equipment for contact sports, building materials, sound proofing materials, seating cushions or automobiles.
A common impact absorption apparatus is a sheet of material having a uniform thickness made of an elastic foam or rubber. A sheet of plastic foam may act as a cushion, absorbing some energy from a load or impact by the compression of the material, so less energy is transmitted through the material to the underlying structure.
There are a number of drawbacks for these impact-absorbing devices. Specifically, these devices typically rely on compression as the most important mechanism for reducing the transmission of force from an impact or load to any underlying structure. Consequently, their effectiveness in absorbing energy from impacts or loads is largely dictated by the thickness of the impact-absorbing material and its elasticity and density.
The required thickness of the impact-absorbing material in a molded piece device gives rise to a number of undesirable properties. When wearing such a device, the wearer's range of motion may be restricted because the thickness of the material required for effective impact-absorption reduces the flexibility of the device. The materials typically used are also typically limited in elasticity, further reducing flexibility. In addition, the thickness and coverage of the molded piece device limits the airflow between molded piece device and the body, causing body heat to be undesirably retained. The ability to make such a device lightweight is also limited by the dependence on the thickness of the material.
Such devices are typically only suitable for a relatively small range of impact forces, as the material will not provide the appropriate resistance outside of that range. Thin low density material does not generally provide sufficient energy absorption in an application where high-energy impacts are expected.
On the other hand, dense materials must be used with caution because of the possibility of injury or damage to the underlying material if they are too dense or rigid. Thus, in many applications, these devices tend to be less flexible or heavier than desirable because of the thickness required for a low enough density material to provide sufficient resistance in the case of an impact.
There are devices that include structural features and the use of composites to absorb energy. For example, there are materials which comprise two impact absorbing materials of different densities that are layered and held together by physical means, adhesives or welds. A softer, lower density material layer may present a more forgiving surface for body contact, while a denser, harder material layer provides more resistance at a reduced thickness. There are also materials that comprise a composite structure having a plurality of cones affixed onto a semi-rigid or rigid substrate of a different material.
While these devices provide some advantages over sheets of plastic foam, these devices rely on compression as the most important mechanism for reducing the force from impacts. Consequently, the effectiveness of these devices is dictated primarily by the thickness of the materials. Since the effectiveness of the devices generally depends on the amount and density of material present in the device, the ability to achieve a lightweight and flexible device is limited.
There is a need for an impact-absorbing structure that is flexible, lightweight and not bulky, and that responds differently depending on the level of force applied.